The invention relates to an apparatus for monitoring a web of material or some other scanning plane relative to faults which influence the remission and/or reflection of a striking beam of transmitted light, whereby a beam of transmitted light scanning the width of a web of material at right angles to its direction of movement is concentrated on the web by a cylindrical lens and light reflected back by the web is projected onto a light guide rod on whose face or faces are provided one or two photosensitive devices.
The object of a simultaneously filed application by the same inventors for APPARATUS FOR MONITORING A MOVING WEB OF MATERIAL FOR FAULTS, the disclosure of which is incoprorated by reference in the present application comprises increasing the significance of the electrical signals obtained, thereby permitting a better differentiation between different types of fault. The parallel application deals substantially with the received beam path and provides an optical arrangement permitting a better classification of the nature of the faults in that both reflected and remitted light are used for evaluation purposes, whereby optionally different spectral ranges can also be used. Thus, a total of two or four electrical signals are available for fault evaluation.